Storm surges are powerful ocean movements caused by wind action and low pressure on the ocean's surface. These types of events can swamp low-lying areas, sometimes for kilometres inland.
Storm surge is an abnormal rise in sea level over and above the normal (astronomical) tide levels. It can be thought of as the change in the water level due to the presence of a storm. These powerful ocean movements are caused by strong winds piling water up against the coast as a cyclone approaches.
Storm tide is the water level that results from the combination of the storm surge and the normal (astronomical) tide. A 3 metre storm surge on top of a high tide that is 2 metres above the mean sea level will produce a storm tide that is 5 metres above mean sea level.
Storm tides can swamp low-lying areas, sometimes for kilometres inland. Strong winds at the coast can also create large waves, worsening the impact.
While significant surges usually accompany tropical cyclones, storm surges caused by large low-pressure systems can also bring dangerous storm tides and damage to coastal communities all around Australia.
Storm surges are at their most dangerous when they arrive at high tide - when the sea is already at its high point. The resulting storm tide can flood inland areas. That's what happened when Hurricane Katrina made landfall in New Orleans in August 2005, resulting in the worst floods - and fatalities - in the city in over 100 years.
The graphic below for TC Vance (1999) illustrates how the normal tide interacts with the storm surge to produce a storm tide.
The graphic shows the actual tide (top line, blue) and the predicted tide (pink) at Exmouth in WA during Tropical Cyclone Vance. The black line is the storm surge – it peaked at 3.5 metres. At the same time the predicted tide was 1.4 metres. So the resultant peak storm tide was 4.9 metres. If the storm surge had occurred at the time of high tide, the actual tide would have been 6.0 m - 2.6 metres above the highest astronomical tide (the highest tide that could occur, shown by the straight blue line).
From this example you can see that the large tidal ranges experienced along most of Australia's tropical coastline mean that the worst impacts will only occur when the storm surge arrives at the time of high tide.
Storm surges are very sensitive to the characteristics of the cyclone, making it very hard to predict. The paths of cyclones are often erratic, making it hard to forecast where and when they will make landfall and how high the tide will be at the time. Other elements contributing to the risk of storm surge include the cyclone's speed and intensity, the angle at which it crosses the coast, the shape of the sea floor and local topography.
These factors all make it very hard to accurately predict the arrival and scale of storm surges. As we have seen above, very large storm tides are rare because they require an intense tropical cyclone to cross the coast at high tide. However, the impact if that does happen can be catastrophic. This is why the Bureau of Meteorology provides information to our emergency management partners to enable them to prepare communities under threat for the "worst case scenario".
Storm surges and Tsunamis are generated by quite different phenomena. While both can cause flooding and significant damage in coastal regions, they have quite different characteristics.
A storm surge is generated by weather systems forcing water onshore over a stretch of coastline. It will normally build up over a time frame of a few hours, as the cyclone or other weather system approaches.
A Tsunami is generated by earthquakes, undersea landslides, volcanic eruptions, explosions or meteorites. These waves travel great distances, sometimes across entire oceans affecting vast lengths of coastal land.